Foraging and Resource Allocation Strategies of Mycorrhizal Fungi in a Patchy Environment

Abstract

Foraging for nutrients and carbon are essential components of the mycorrhizal symbiosis. Foraging strategies of mycorrhizal fungi have received little attention compared to the interaction with the plant. Proliferation of hyphae, resource allocation (carbon and nutrients) within a mycelium and spatial distribution of resource capturing structures (internal mycelium for carbon and external mycelium for nutrients) can be considered as foraging strategies. The arbuscular mycorrhizal fungi (AMF) form a uniformly distributed mycelium in soil, but hyphal proliferation occurs in response to several types of organic material and near potential host roots. The ecto-mycorrhizal fungi (EMF) normally form denser hyphal fronts than AMF, and they respond to both organic material and inorganic nutrients by increased growth. This is especially evident for the EMF that form extensive mycelia connected by differentiated hyphal strands, so-called rhizomorphs. We hypothesise that the growth strategy of the AM fungal mycelium reflects an evolution towards optimal search for potential new host roots. The growth strategy of EMF instead seems to reflect evolution towards optimised nutrient capture in competition with other mycelia. Foraging behaviour of mycorrhizal fungi is discussed and we suggest two conceptual models for resource allocation in the mycorrhizal mycelium. These models consider both the internal and the external mycelium and the trade-offs between different foraging strategies of mycorrhizal fungi. From the experimental data available, it is clear that mycorrhizal fungi forage. It needs to be investigated whether observed foraging strategies are optimal for the mycelium as one individual.